沖切斷面特徵對沖壓成形邊緣破裂影響之實驗分析

Abstract

隨著科技進步以及對環保的重視，汽車生產時對車體結構的要求日益提升，同時也對汽車安全考量的逐漸重視，因此各大車廠將增加車體強度與降低車體重量作為努力的目標。其中先進高強度鋼具有較高的強度及良好的塑性，且已被廣泛應用於汽車結構件。然而先進高強度鋼在沖壓成形上容易產生邊緣破裂，其尚難以透過有限元素法模擬進行預測，且目前用於預測沖壓板材是否發生破裂現象的成形極限曲線無法有效預測邊緣破裂發生。因此為了解決邊緣破裂現象，本論文將透過實驗分析方法探討沖切斷面特徵對邊緣破裂的影響。 本論文首先針對邊緣破裂的成因進行文獻收集與歸納，了解影響邊緣破裂時機的原因非常複雜，其中先進高強度鋼在微觀結構上容易產生微孔破壞且板材邊緣在不同沖切製程下，將有不同的邊緣破裂現象。此外，也得知擴孔試驗與圓孔拉伸試驗的局部成形性可有效對應到邊緣破裂的變形機制，藉由本論文所進行的一系列相關實驗，後續可透過圓孔拉伸試驗建立破壞準則的探討。 在圓孔拉伸試驗以及有限元素法模擬模型建立，本論文已獲得不同材料在不同沖孔條件下的最大主應變，並將其定義為破壞準則，同時也確定了不同沖孔條件下局部成形性的趨勢。在沖孔實驗中，分別觀察不同材料沖切斷面特徵的趨勢，同時與圓孔拉伸試驗所得的局部成形性趨勢進行比較，已得知斷面邊緣的硬度差與局部成形性呈現負相關，最後透過硬度與應力的正比關係，在建立的沖孔模擬模型也可觀察到應力有相同的變化趨勢。未來可透過本論文所建立的破壞準則與模擬模型，預測實際載具邊緣破裂的時機以及作為改善沖切製程的參考。

With the advancement of technology and concern for environmental protection, the requirements for fuel-efficient and lightweight automotive structures and the attention to automotive safety have been increasing. Therefore, increasing the strength and reducing the weight of the car body has become the goal that manufacturers put effort into. Advanced high-strength steel has an excellent combination of strength and plasticity and has been widely used in automotive structural parts. However, advanced high-strength steels are prone to the occurrence of edge cracks during stamping, which is still difficult to be predicted through finite element method simulation. The forming limit curve that is usually used to predict forming failure cannot effectively predict edge cracking. Therefore, in order to understand the phenomenon of edge cracking, this thesis will explore the influence of the sheared cross-section characteristics on edge cracking through the experimental analysis method. This thesis starts by collecting the documents on the causes of edge cracking and summarizes that the mechanism which affects the timing is complicated. In addition to the micro-structures that are prone to generating micro-voids in AHSS, the edge of the sheet will have different timings of edge cracking under different shearing processes. Furthermore, it is also known that the local formability of the hole tensile test and the hole expansion test can correspond to each other effectively, so that the failure criterion can be established through the hole tensile test. The related experiments are conducted in this thesis. By the establishment of the experiment and finite element method simulation model of hole tensile test, this thesis has obtained the maximum principal strain of different materials under different punching conditions and defined the maximum principal strain as the failure criterion, which also determined the local formability under different punching conditions. In the punching experiment, the trend of punching cross-section characteristics of different materials was observed respectively, which are compared with the trend of local formability obtained by the hole tensile test. It is known that the hardness difference of shear-affected zone has negative correlation with the local formability. Through the proportional relationship between hardness and stress, it can be observed that the stress has the same changing trend in the established punching simulation model. In the future, the failure criterion and simulation model established in this thesis can predict the timing of edge crack happening on real-case stamped part and also become a reference for improving the punching process.